World War I was caused by tensions that had gripped Europe for years. Prior to 1914 there had been many changes effected in European geography. Beginning in the 1860s Prussia, in a series of short wars, began to conquer the separate Germanic states. Prussia then defeated Denmark; furthermore, with its Germanic tribes and Denmark under its rule, Prussia, was able to defeat France, thus chaninging the political geography of Europe.

Following the Franco-Prussian War, the Germanic states unified their twenty-six independent states into the German Empire, declaring Prussian King Wilhelm I the Kaiser and Bismarck the chancellor [like a prime minister]. Wishing to penalize France for its recent defeat, Germany claimed Alsace-Lorraine for its own and forced the French to pay reparations. This act led to implacable hatred between France and Germany.

Hoping to avoid having to fight both France and Russia, Germany sought to form an alliance with Russia. The emperors of Germany, Austria-Hungary, and Russia pledged friendship to one another in the Reinsurance Treaty of 1887; however, this did not last after the Kaiser died and Bismarck, who was architect of the peace, left office. Relations soon broke down, and Russia formed an alliance with Germany's enemy, France.

When an Austro-Hungarian Empire was formed, it was destined to failure as the Hungarian Empire was composed of ethic groups left over from the old Ottoman Empire that had extended across Europe. Consequently, the ethnic divisions among Serbs, Slavs, Croatians, Romanians and Austrians led to the break-up of this empire. Here conflicts developed from changes made in geography that had moved peoples from their original homelands.

Then, too, across the English Channel, lay the country of Great Britain, a puissant country with the greatest empire in the world. Certainly, Britain did not want Russia or Germany or France to expand their empires, so it kept watch over them and protected its own geography by allying itself with Russia and France.

I would say that human geography played the largest role in the outbreak of WWI. Specifically, WWI was caused in large part by the fact that some ethnic groups were being ruled by people of other ethnic groups.

The war started when Slavic nationalists assassinated the Archduke of Austria-Hungary. The ruling family of that empire was ethnically German, but many of its subjects, especially in the Balkans, were Slavs.

When Austria-Hungary tried to punish Serbia (a Slavic country) for its role in the killing, Russia (another Slavic country) came to Serbia's aid. Then Germany came to Austria-Hungary's aid for ethnic reasons as well.